To meet customer demand, the commercial printing industry requires the capability of producing spot colors and color images accurately and consistently. In a typical four color CMYK printer, when rendering a given color (Lab) on a CMYK printer, such as for spot color emulation, there is a range of CMYK values that will produce the desired Lab value. The available CMYK range is large for some colors (e.g., mid-tone neutrals) and small or zero for others (e.g., saturated colors). Although each CMYK value in the range will produce the desired Lab value, the printed spot colors with that recipe can differ widely in other attributes, such as graininess, mottle, color stability, ink cost, etc. Consequently, it is desirable to pick the CMYK recipe from among available recipes to optimize image quality. Methods for performing this optimization are computationally intensive since such techniques involve computing all possible CMYK recipes for a given color and then selecting among these. Depending on the choice of CMYK recipe, appearance of spot colors may be noisy due to the differences in uniformity and in contrast between the various separations, for example. In spot color tests on several printers, proper selection of the C, M, Y and K separations has been shown to suppress the noise apparent in the prints, making them appear smooth. An optimized CMYK recipe is considered useful when it not only produces accurate color but also renders colors that appear smoother (less noisy).
Prior art methods use open-loop multi-objective optimization which may or may not contain optimal values. While such a use is known, there is a need for a feedback-based approach which simultaneously minimizes multiple image quality attributes. Although the CMYK recipe and process actuators can be simultaneously searched in order to improve image quality in color rendition, optimization can only be performed on a single color at a time. As such, there is a need to setup the process actuators to an optimal image quality attribute set point so that a majority of the CMYK colors are printed near optimal for a selected image quality attribute.
Accordingly, what is needed in this art is a feedback control loop which simultaneously minimizes image quality attributes for selected colors of interest.